Situation

Mobile code is faced with the problem of heterogeneous hosts. Hosts differ in hard- and software configuration. A first approach is a common base on every host, that provides a homogeneous computational environment to the mobile code, e.g. Java [#!LiYe1999!#]. The homogeneous interface of the computational environment is achieved by abstraction from the environment-dependent details. For instance, Java abstracts the file system and offers a platform independent API for accessing its resources. Because platforms strongly differ it is not possible to abstract the entire functionality, which constrains the abstraction to a subset of what is available. The approach of abstraction works fine as long as the highest common denominator is sufficient to the application. If the demands of the application can not be satisfied by the platform independent computational environment, it is necessary to add environment dependent implementations to the application. The application has to decide when to use the right implementation.